This invention relates to electrochemical measurement and detection. More specifically, it relates to an improvement in the use of a solid electrolyte and a catalyst in detecting the presence of hydrogen or gases capable of dissociating to yield or combining with hydrogen ions, including oxygen, and measuring the quantity present. The improvement involves the use of a substance in solid form in place of a reference substance in gaseous form.
The use of solid electrolyte sensors for detecting oxygen, particularly in automotive exhaust gases, is well known. The present invention utilizes similar basic principles for detection of certain gases. The Nernst equation describes the behavior of sensing devices using solid electrolytes. When two media with different partial pressures, P.sub.1 and P.sub.2, of a particular substance present in both media are separated by a solid electrolyte (ionic conductor) and conducting electrodes are attached to both sides of the ionic conductor, an EMF is generated which is related to the partial pressures as follows: ##EQU1## where R is the gas constant, T is absolute temperature, F is the Farady constant, E.sub.o is the standard oxidation-reduction potential difference, EMF is electromotive force, and n is the number of electrons per molecule of product from the overall cell reaction. If the system described by the above equation behaves non-ideally, the partial pressures must be replaced by fugacities. Another factor which may need to be considered in regard to a particular system is the rate of dissociation to form the ions which pass through the solid electrolyte. This may be a limiting factor to the transfer of ions through the electrolyte. The rate of dissociation can be calculated by means of the equilibrium constant for the dissociation reaction.
A novel solid electrolyte membrane is used in the present invention. We have discovered that a polymer blended membrane may be fabricated by admixing a heteropoly acid or a salt thereof with an organic polymer which is at least partially compatible with said heteropoly acid or salt to form a polymer blended composition of matter which is useful in gas detection. It was totally unexpected that a thin film membrane could be cast from such a blend to provide a membrane which would be highly selective to certain gases and therefore able to act as a proton conductor in a hydrogen detector where molecular hydrogen is converted into protons on one side of the device, transported through the membrane, and recombined as molecular hydrogen on the other side. The membrane is also useful in detecting gases capable of dissociating into or combining with hydrogen ions. Also used in the invention is a solid substance which is a substitute for a reference gas, which reference gas is one of the two media mentioned above in the discussion of the Nernst equation. It is highly desirable to use a solid reference substance, which requires only periodic replacement, instead of maintaining a continuous reference gas flow, or in appropriate situations, maintaining a sealed chamber of reference gas. For background information relating to the present invention, reference may be made to the book Solid Electrolytes and Their Applications, edited by Subbarao, Plenum Press, 1980.